I'm having trouble getting full torque out of a DC motor at anything other than full speed with a PWM driving circuit.
Assuming the problem is inductance of the coils, what can be done besides decrease the driving frequency?
What is usually done? Nothing? If something, what?
I was thinking of using a constant current source, but at the beginning of each pulse, this would apply a voltage across the coils higher than the rated voltage. So, two questions:
Is applying a higher voltage OK? Or would it have to be a constant power source, rather than a constant current source? Or despite the counter-emf of the coil, is any voltage applied beyond specs going to reduce life or safety of the motor? And would the additional circuits have to be active, or would some clever passive circuit increase the voltage when each pulse energises the coils, without even needing a higher voltage power supply?
Best Answer
@Mark is correct, you can run from a higher voltage if it is available. This will mean running with PWM all of the time.
However, if you are wanting to run near full torque and still be able to run at the top speed at a given voltage, try using a fixed "off" time in your PWM instead of playing with the duty cycle. Trigger your "off" pulse from a threshold against your current sense. When operating properly, the number of pulses per cycle will decrease as you go and you will be able to get down to a single pulse per cycle. During the "off" cycle, switch off the high side driver and switch on the complementary low-side driver to give all that current someplace to go. You can narrow down the fixed pulse width and get as close to full torque as you need.
Be aware that with this approach, the motor will run at full speed until the load is applied, then it will run at your specified torque. So it is a torque control rather than a speed control.